Typically in radio frequency (RF) communications systems, it is advantageous to use a balanced antenna. A balanced antenna reduces the RF current on the housing and other parts of the radio equipment, and the antenna is less susceptible to being detuned or being blocked by the operator. When connecting a balanced antenna with an unbalanced RF circuit the interface between the antenna and the unbalanced circuit requires a device called a balun.
In circuit technology, an unbalanced system (FIG. 1) is defined as one in which two conductors are at different potentials with respect to ground. One of the conductors is often at the ground potential. The capacitance with respect to ground of each of the two conductors is then different, consequently, the current in the two conductors may be different. A balanced system is one in which the potential of each of the two conductors are respectively above and below ground potential by the same magnitude. FIG. 1 illustrates a balun that electrically connects an unbalanced electrical source or lead and a balanced electrical source or load. FIG. 2 is an illustration of a simplified model of how currents are defined by a balanced and an unbalanced mode. In a RF transmit communication system, the source is the transmitter and the load is the antenna. Any configuration of currents Ia and Ib can be expressed as a combination of common mode and differential mode currents. Both the common mode and differential mode currents are determined by currents generated by either a balanced or an unbalanced source. The common mode current, shown as ICa and ICb in FIG. 2, have equal magnitude and equal phase. Consequently, the common mode currents contribute nothing to the intended operation of the load, or antenna, and are usually dissipated in heat. The differential mode currents, IDa and IDb, are equal in magnitude and opposite in phase, consequently, they manifest the power into the intended load. The source and the load losses of the common and the difference or differential modes can be represented as a circuit, as shown in FIG. 3. For balanced loads such as balanced antennas, the intended or desirable mode is the differential mode, and the unintended or undesired mode is the common mode. By maximizing the impedance of the common mode load, the currents and the loss associated with the common mode currents will be minimized.
A balun permits connection between a balanced system and an unbalanced system in such a way that the potentials to ground, and the currents in the two parts of the balanced structure are equal in magnitude and opposite in phase. In the past, balun transformers and transmission lines or "bazooka" baluns have been used to perform the balun function for an antenna feeder in a communication device used in a RF communication system. A balun transformer effectively performs the balun function, however, for use in such devices as portable radiotelephones, a balun transformer is large and absorbs power. Typically about 0.7 dB of power is lost through a balun transformer, thus, significantly reducing the amplitude of signal transmitted between the transceiver and an antenna. Second, "bazooka" balun, or transmission line, requires more than two conductors, or two conductors and a sleeve about those two conductors to perform the balun function. This "bazooka" balun requires very large physical space for a sleeve within a communication device.
Often, communication devices, such as a portable radiotelephone, are required to be physically small and less power-consuming than other non-portable or stationary communication devices. Thus, it is desirable in a portable radiotelephone to have an efficient transfer of power between the transceiver and the antenna, and to have a small physical size. Thus, it would be desirable to have an efficient and small balun device for transferring signals between a balanced antenna and unbalanced circuitry within a transceiver in a communication device.